Table 3 (continued )
In conclusion, a novel Lewis acid-catalyzed oxidation of
benzylamines to the corresponding amides using TBHP as the
terminal oxidant has been developed. In the presence of
10 mol% of ZnBr2 or FeCl3, in 0.1 mL of pyridine at 80 1C,
not only secondary amines, but also useful primary benzylamines
can be applied as substrates. Both electron-donating and
electron-withdrawing aryl groups such as aryl halides and
picolines are tolerated. With respect to the reaction mechanism,
it is under investigation in our laboratory.
Entry Benzylamine
Product
Yieldb [%]
13
81
a
Benzylamines (1 mmol), ZnBr2 (10 mol%), pyridine (0.1 mL), TBHP
b
(tert-butyl hydroperoxide) (3 mmol), 80 1C, 16 h. Isolated yield.
Pyridine (2 mL). FeCl3 (10 mol%), yield was determined by GC
c
d
e
using hexadecane as internal standard based on benzylamines. FeCl3
(5 mol%) was used.
Notes and references
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¨
With convenient reaction conditions in our hands, we
started the evaluation of the generality and efficiency of the
reaction using different substrates (Table 3). 59% of N-methyl
benzamide was isolated under our standard conditions
(Table 3, entry 1). 3-Methyl-substituted N-methyl benzamide
was formed in 39% yield, which increased to 59% when
carrying out the reaction in 2 mL of pyridine (Table 3, entry 2).
4-Bromo-substituted N-methyl benzamide was produced in 48%
yield as well (Table 3, entry 3). Again, in these cases the corres-
ponding benzaldehydes and benzoic acids were also detected.
N,N-Dimethyl benzylamine could be applied as a substrate as well,
10% of N,N-dimethyl benzamide was isolated together with
certain amounts of benzaldehyde. In addition to the secondary
benzylamines, primary benzylamines were successfully applied as
substrates under our standard conditions.
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In comparison with secondary amines, primary amines that
are more easily available led to higher yields of useful primary
amides. Hence, 38% of benzamide was produced from benzyl-
amine in 0.1 mL of pyridine, which was improved to 50%
when using 2 mL of pyridine and to 85% when using 10 mol%
of FeCl3 as the pre-catalyst (Table 3, entry 4). tert-Butyl-,
methyl-, and methoxy-substituted benzamides were isolated in
moderate yields, which are all potentially possible to be
improved by carrying out the reactions in 2 mL of pyridine
or using FeCl3 as catalyst (Table 3, entries 4 and 5). Naphthalen-
1-ylmethanamine can also be applied as a substrate and gave the
corresponding primary amide in 72% yield (Table 3, entry 8).
Additionally, 66% of 4-fluoro- or 4-chloro-decorated benzyl-
amides were isolated by the oxidation of their corresponding
benzylamines (Table 3, entries 9 and 10). Remarkably, picolyl-
amine derivatives were successfully oxidized to the corresponding
primary amides in good isolated yields without further optimiza-
tions (Table 3, entries 11–13). Finally, 2-phenylethanamine and
butylamine were tested under our standard reaction conditions
as well. Here, 15–20% of the desired primary amides were
isolated with significant amounts of aldehydes, acids, and nitriles
being formed, too.
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 12237–12239 12239